Shielded coaxial cable transformer

ABSTRACT

A coaxial cable transformer which includes a shielded conductor for reducing primary to secondary winding capacitive coupling which results from the mutual capacitance therebetween. Included between and concentric with inner and outer coaxial conductors, operating as primary and secondary windings, is a selectively grounded shield conductor. This shield conductor is grounded such that there is no instantaneous potential difference between corresponding points on the shield and secondary winding. To reduce indirect primary to secondary capacitive coupling which results from capacitive current between the primary and shield conductors, additional capacitance is included in the transformer circuit.

United States Patent 1 Horna [54] SHIELDED COAXIAL CABLE TRANSFORMER[75] Inventor: Otakar Antonin llorna, Bethesda,

[73] Assignee: Communications Satellite Corporation, Washington, DC.

[22] Filed: May 19, 1971 [21] Appl. No.: 144,940

[52] US. Cl ..323/44 R, 323/48, 336/69, 336/84, 336/195 [51] int. Cl.1.110115 27/36 [58] Field of Search....323/44 R, 48; 336/69, 84, 195

[56] References Cited UNITED STATES PATENTS 1,320,980 11/1919 Bowman..336/84 3,244,960 4/1966 Stevens et al....

2,553,324 5/1951 Lord 1,362,138 12/1920 Pratt ..336/84 [451 Feb. 20,1973 2/1942 Grimditch v.336/84 X 11/1955 Hayes etal. ..336/84 X PrimaryExaminerA. D. Pellinen AttorneySurhrue, Rothwell, Mion, Zinn & MacPeak[57] ABSTRACT A coaxial cable transformer which includes a shieldedconductor for reducing primary to secondary winding capacitive couplingwhich results from the mutual capacitance therebetween. Included betweenand concentric with inner and outer coaxial conductors, operating asprimary and secondary windings, is a selectively grounded shieldconductor. This shield conductor is grounded such that there is noinstantaneous potential difference between corresponding points on theshield and secondary winding. To reduce indirect primary to secondarycapacitive coupling which results from capacitive current between theprimary and shield conductors, additional capacitance is included in thetransformer circuit.

5 Claims, 10 Drawing Figures YPATENTEDFEBZOM v v 3717.808

sum 20F 3 PRIOR ART SHIELDED COAXIAL CABLE TRANSFORMER BACKGROUND OF THEINVENTION An important requirement of a high-frequency transformer isthe generation of an output signal which corresponds as precisely aspossible to the input signal,

save for amplitude distinctions resulting from the primary to secondaryturns ratio. As a result of stray capacitance and inductance intransformer circuits, output signals often appear distorted. In pulsetransformers this distortion appears primarily as a distorted transientresponse. Transient distortion is seen as a slow rise time along with aringing or oscillatory transient portion of the output pulse.

In prior transformers with non-concentric windings, the primary cause oftransient distortion was stray inductance, stray capacitance beingnegligible. Development of coaxial cable transformers such as thosedescribed in U.S. Pat. No. 3,005,965 and US. Pat. No. 3,197,723resulting from the realization that stray inductance could beappreciably reduced by forming the primary and secondary windings fromconcentric conductors wound on a suitable core.

Although coaxial cable transformers did indeed reduce stray inductance,the close proximity between the primary and secondary windings gave riseto an appreciable stray capacitance caused by the mutual capacitancebetween these windings. It is the object of this invention to reducethis capacitive coupling between the primary and secondary windings.

SUMMARY OF THE INVENTION In accordance with the teaching of thisinvention transient distortion in coaxial cable transformers issubstantially eliminated by reducing the capacitive current whichresults from the mutual capacitance. Reduction of capacitive currentreduces the transient distortion. To reduce capacitive current in thesecondary circuit a third conductor is located between first andsecondconcentric conductors, functioning as primary and secondary windings.This third or shield conductor is selectively connected toa referencepotential so that corresponding points on the shield and secondaryconductors have the same instantaneous high-frequency potential withrespect to the reference potential. As a result, no capacitive current,which results from an instantaneous potential difference betweenadjacent windings, flows through the mutual capacitance between theshield conductor and the secondary.

The shield conductor may give rise to an indirect capacitive couplingbetween the primary and secondary windings. This coupling results fromthe instantaneous potential difference between the primary and theshield conductor which causes capacitive current to flow in the shieldconductor. This capacitive current can give rise to an inducedcapacitive current in the secondary. To eliminate this indirectcapacitive coupling, the invention provides for the addition of ,acapacitor between the primary and the shield conductors, when needed, toassure that the resultant potential induced in the secondary in responseto the shield conductors capacitive current is substantially zero.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents an equivalentcircuit for a highfrequency transformer,

FIG. 2 represents the transient portions of input and output pluses froma coaxial cable transformer without the improvements of the presentinvention;

FIG. 3a and 3!: illustrate a transformer constructed according to theteaching of this invention,

FIG. 4 illustrates a prior art current transformer coupled to a circuitin which it operates.

FIG. 5 illustrates the circuit of FIG. 4 modified to include theteachings of this invention,

FIG. 6 illustrates the circuit of FIG. 5 further modified to include acapacitor between the primary and shield conductor,

FIGS. 7a and 7b illustrate a transformer constructed according to theteachings of this invention and including a shielding box surroundingsubstantially all of the transformer, and

FIG. 8 illustrates a transformer embodiment built ac- I cording to thisinvention and providing a 1:2 turns ratio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION FIG.1 represents an equivalent circuit for a highfrequency transformer validfor concentric and nonconcentric winding transformers. This equivalentcircuit will be used to explain the operation of the invention.

.The primary of the transformer of FIG. I is coupled at points a, b tosource E containing an internal resistance R Source E drives the primarywinding to induce a potential across the secondary to drive the load RThe primary inductance of the circuit given by L The stray inductance onthe primary side is given by L, and is represented by two inductors inseries with the inductance L The stray inductance produced on thesecondary side of the transformer is given by L and is represented inthe equivalent circuit by two inductors in series with the windingcapacitance C Capacitance C represents the capacitance between adjacentturns of the secondary winding, while capacitance C represents thecapacitance between adjacent turns of the primary. Since each secondaryturnv in a coaxial cable transformer is shielded by the outer braid, thevalue of C), is negligible. The mutual stray capacitance, hereinafterreferred to simply as the mutual capacitance, between the primary andsecondary windings is distributed along the length of the windings.

This distributed capacitance is represented in FIG. 1 by v a pair oflumped capacitive elements C In practice C C For ease in discussing thetheory behind the invention a capacitance C' is defined as theequivalent stray capacitance as viewed from the primary side of thetransformer where C' kC C C representing all the stray capacitanceexcluding the mutual capacitance. Since the value of C for coaxial cabletransformers is negligible compared to the value of kC the equivalentstray capacitance is essentially equal to the equivalent mutualcapacitance and therefore C' kC The value of the constant ofproportionality, k, depends on the circuit in which the transformer isused and is primarily dependent upon the transformer turns ratio and thetransformer-load interconnection. For example, if points b and d in FIG.1 were grounded or more generally connected to the same high frequencyreference potential, the equivalent mutual capacitance would beconsiderably lower than if points b and c were coupled to the same highfrequency reference potential. Such variations are reflected in thevalue of k. At this point it is noted that the term ground will be usedherein to denote any suitable common high frequency reference potential.

It has been determined that as C 3 increases so does the transientdistortion.

A better appreciation of the transient distortion may be had byreferring to FIG. 2. Waveform or represents the transient portion of anessentially ideal pulse. It is not ideal since it shows a finite risetime. The transient portion of the corresponding output pulse isillustrated by waveform B. The transformer producing this pulse does notcontain the elements of this invention. The IO%/90% rise time for theideal pulse is represented by time t,.,. The rise time for the outputpulse is given by t,.,. The difference between t,. and t,., is the risetime distortion caused by the stray capacitance and inductance. Ringingdistortion, shown as theoscillatory portion of waveform B also resultsfrom the stray inductance and capacitance.

In transformers with primary and secondary windings not formed fromconcentric conductors, the relatively large distance between thewindings results in a negligible stray capacitance. Therefore, thetransient distortion is primarily a function of the stray inductance.This 'can be seen from the following equation. With stray capacitanceneglected the %/90%' rise time t, is expressed as:

t 2.2 (L R R") vantages realized with lowering L Therefore, it b'ecomesnecessary to reduce the effect ofthis equivalent capacitance C; withouteffecting the value of L This is done by controllingthe capacitivecurrent flowing in the secondary which results from C Capacitive currentin the secondary is controlled by providinga third or shield conductorgrounded so that each point on the shield conductor has the sameinstantaneous high-frequency potential with respect to ground as thecorresponding point on the secondaryconductor. Under this condition,there is no instantaneous voltage difference between correspondingpoints on the shield conductor and the secondary winding, therebypreventing a capacitive current between the primary and secondarywindings.

FIG. 3a illustrates the basic configuration of a transformer designed inaccordance with the teachings of I this invention, while FIG. 3billustrates the FIG. Baconfiguration in schematic form. The threeconductor coaxial cable is wound around a core 1. The outer conductor 2'which may 'be used as-the primary winding cylindrically encloses and isisolated from theshield conductor 6 by suitable insulating material 8.Similarly, inner conductor 4 is isolated from the shield conductor 6 byinsulating material 10. When outer conductor 2 acts as a primarywinding, conductor 4 acts as the secondary. The shield conductor isprovided with a terminal o for coupling the shield to ground at onepoint only. The position of terminal g on conductor 6 is dependent uponthe circuitry coupled across terminals c, d. In every case, however,terminal g is positioned so that corresponding points on the shield andsecondary conductors have the same instantaneous high-frequencypotential with respect to ground. Therefore, there is no instantaneouspotential difference between the shield and the secondary winding andthus no capacitive current flow.

The rules for selectively grounding the shield conductor may better beexplained with reference to a circuit which includes a coaxialtransformer. FIG. 4 shows a coaxial cable transformer connected as acurrent transformer. The primary of this transformer is connected tosource 12 at terminal a and to load 14 at terminal b. Terminal c of thesecondary is connected to ground through a load resistor R, whileterminal d is connected to ground through load resistor R Thistransformer is not constructed in accordance with the L teaching of thisinvention.

Capacitance C and C represent winding 4. At this point it is notedthatcommon numericals designate equivalent elements in the differentfigures. If resistance R, and R, are equal, then in the absence ofcapacitive current,-the voltages across resistances R, and R, are equalin magnitude, but of op-.

posite instantaneous polarity with respect to ground. If capacitivecurrent, 'i flows through the mutual capacitances C and C the voltagesacross the re sistances have additional components,'i,R, and i R whichhave the same instantaneous polarity with reference to ground causingv,,,,\# -v,,,, Y.

The generation of this capacitive current flow across each of theresistors. Thus, by way of explanation only and with no intent to solimit the invention, terminal c may be at +0.5 voits with respect toground in'which case terminal d would be at O.5 volts with respect toground. However, terminals a, and b are both.

at approximately 1,000 volts with respect to ground giving rise to apotential difference between terminals a.

and c and b and d., Of course corresponding potential differences appearbetween other corresponding points on the primary and secondarywindings. This potential difference causes the flow of capacitivecurrent through the mutual capacitance, illustrated as C and C causingtransient distortion. Thus in our illustrative example, as can be seenfrom FIG. 4, the voltage drop across resistor R, due to the capacitivecurrent i is in a mutual capacitance between primary winding 2 andsecondary direction that causes it to increase the absolute potential ofv,, such that l v,, a |0.s are, [while b. a third conductor locatedconcentric with and between said first and second conductors, and

c. couplingmeans coupling said third conductor to a reference potentialwhereby the instantenous potential at corresponding points on the secondand third conductors are equal so that capacitive current flow in saidsecond conductor resulting from the mutual stray capacitance betweensaid first and second .conductors is substantially reduced.

FIG. 5 illustrates the circuit of FIG. 4 modified to include atransformer constructed in accordance with the teachings of thisinvention. This transformer includes a selectively grounded shieldconductor 6 surrounding inner conductor 4 which is functioning asasecondary winding. Mutual capacitance exists between the primarywinding 2 and the shield conductor 6 as well as between the shieldconductor 6 and conductor 4, as illustrated by capacitances C C and C C,respectively.

-To eliminate direct capacitive coupling between the shield conductor 6and winding 4 there must be no instantaneous voltage difference betweenthe windings. This is accomplished in accordance with the teachings ofthis invention by selectively locating terminal g on winding 6 andcoupling that terminal to ground. With resistors R and R assumed equaland with the winding resistance of conductor 4 distributed uniformlyover its length, the midpoint between terminals c and d is at groundpotential. Therefore, the terminal g is located at the mid-point ofconductor 6 and then connected to ground. Since points e and fare at thesame instantaneous potential with respect to ground as points c and drespectively, corresponding points on the shield and secondaryconductors have the same instantaneous potential. If resistors R and Rare unequal then a point other then the midpoint of conductor 4 is atground potential. In general, the ground point on conductor 4 for theconfiguration shown is determined by the ratio of R,:R,. Therefore thelocation of terminal 8 is such that pl m R,:R,, where rt and 2,,represent the number of turns between points e, g and g, f respectively.

Introduction of the shield conductor 4 results in a possible indirectcapacitive coupling between the primary and secondary windings. Thisindirect coupling occurs because of the potential difference between theprimary and shield conductors. As a result, capacitive current flowsthrough the mutual capacitance represented in FIG. 5 by capacitives C,and C This capacitive current in winding 6 induces a voltage which bytransformer action appears in secondary winding 4 giving rise tocapacitive current in'the secondary circuit.

A technique for eliminating this indirect capacitive coupling will nowbe explained. For the circuit of FIG. 5, terminal g was positioned atthe midpoint of conductor 6. Therefore, the voltage across capacitors Cand C is V the source voltage, causing the capacitive currents i and ito be of the same magnitude but of opposite direction and thus theireffect is suppressed. That is, the potential induced in conductor 4 as aresult of i and i are equal in magnitude but opposite in polarity. WhenR does not equal R the positioning of .the terminalg on conductor 6 ischanged to assure that the instantaneous potential difference atcorresponding points on the shield and secondary conductors is zero. Aspreviously explained, this requirement is met by positioning terminal 3such that the ratio n,,:n,, R,:R, In such a case, the capacitive currentinduced potential in .winding 4 does not cancel and a capacitance causedcurrent flows in the secondary circuit.

FIG. 6 representsan embodiment of the invention incorporating means forcompensating for this capacitive current. The potential across conductor4 due to c apacit ive currents i and ii is given by theexpression V n ni where n and n,; represent the number of turns between points eg and gfo r conductor 6 respectively. Therefore, if egeg n,,, Vale can bereduced by increasing i Since i C dV /dt, i can be increased byincreasing C In practice this is achieved by increasing the capacitancein the vicinity of the turns n The added capacitance is illustrated inFIG. 6 by capacitor C Since conductor 6 protects against direct couplingbetween the primary and secondary windings, an increase in thecapacitance between the primary and shield conductors has no effect uponthe secondary circuit.

FIG. 7a illustrates another transformer arrangement which incorporatesthe teachings of this invention. This embodiment further protectsagainst the introduction of capacitance induced current in the secondarycircuit by enclosing the transformer in a shielding box.

FIG. 7b is a schematic drawing of the FIG. 7a transformer configuration.The split primary configuration illustrated in FIG. 7b is conventional.In such transformer configurations equal and opposite pulse trains areapplied to the primary side of the transformer to produce output pulseshaving twice the amplitude as the input pulses. However, in accordancewith the teachings of this invention. the transformer whichincludes'shield conductor 6 is surrounded by a shielding box 20.Terminals a and b, which receive input signals are connected to primaryconductor 2 through the shielding box 20 by means of coaxial connectorsshown diagrammatically at 7. Terminals h and i are connected directly tothe shielding box 20 by any suitable means. Forexample, these terminalsmay be soldered to the box 20. Openings are made in the box to permitpassage of conductors 4 and 6. With this configuration terminals a and bare capacitively shielded from terminals c and d, thus further reducingsecondary circuit capacitance induced current.

FIG. 8 illustrates the applicability of this invention to a coaxialtransformer built with a primary to secondary turns ratio 'other than1:1. Again, shield conductor 6 is selectively grounded at one point onlyso that there is no instantaneous potential difference betweencorresponding points on conductors 6 and 4. In other respects theconfiguration of this coaxial cable transformer is conventional.

Although the invention has been described with respect to the preferredembodiment thereof, it is to be understood by those skilled in the artthat various modifications can be made in construction and arrangementwithin the scope of the invention as defined in the appendant claims.

What is claimed:

1. In a transformer circuit including a source and load impedance, acoaxial cable transformer comprisa. first and second concentricconductors, said first conductor cylindrically enclosing said secondconductor, said first and second conductors functioning as primary andsecondary windings, the voltage drop across resistor R due to thecapacitive current i is in a direction that causes it to decrease theabsolute potential of V such that V,,,i -0.5 i R i Thus, i V V Thisvoltage imbalance gives rise to transient distortion of the output.

2. The transformer circuit of claim 1 further including appliedcapacitance between said first and third conductors to substantiallyeliminate the resultant capacitive current in said third conductorcaused by the mutual stray capacitance between said first and thirdconductors.

3. The transformer circuit of claim 1 further comprising shielding meanssurrounding said transformer circuit and electrically connected to oneend of said first conductor and said reference source, and connectormeans for permitting electrical connections through said shielding meansto said circuit.

4. The transformer circuit of claim 3 further comprising a fourthconductor surrounding said third conductor, one end of said fourthconductor being electrically connected to said shielding means and meansfor connecting the other ends of said first and fourth conductor to saidconnector means.

5. In a transformer circuit including a source and load impedance, acoaxial cable transformer comprisa. first and second concentricconductors said first conductor surrounding said second conductor, saidfirst and second conductors functioning as primary and secondarywindings,

. a third conductor located concentric with and between said first andsecond conductors,

. coupling means coupling said third conductor to a reference potentialto cause the instantaneous potential at corresponding points on thesecond and third conductors to be equal so that capacitive current flowin said second conductor resulting fromthe mutual stray capacitancebetween said first and second conductors is substantially reduced, and

. wherein the primary winding of said coaxial cable transformer isserially connected between said source and load, further including asecond load impedance coupled between one end of the secondary windingof the transformer and said reference potential and a third loadimpedance coupled between the other end of said secondary winding andsaid reference potential, said coupling means being connected to saidthird conductor at a point along the length of said conductor such thatthe ratio of the number of turns of said third conductor between saidone end and said coupling means to the number of .turns of said thirdconductor between said other end and said coupling means equals theratio of the values of said second load impedance to the said third loadimpedance.

1. In a transformer circuit including a source and load impedance, acoaxial cable transformer comprising: a. first and second concentricconductors, said first conductor cylindrically enclosing said secondconductor, said first and second conductors functioning as primary andsecondary windings, the voltage drop across resistor R2 due to thecapacitive current ic is in a direction that causes it to decrease theabsolute potential of Vdg such that Vdg -0.5 + icR2 . Thus, Vcg Vdg .This voltage imbalance gives rise to transient distortion of theoutput.
 1. In a transformer circuit including a source and loadimpedance, a coaxial cable transformer comprising: a. first and secondconcentric conductors, said first conductor cylindrically enclosing saidsecond conductor, said first and second conductors functioning asprimary and secondary windings, the voltage drop across resistor R2 dueto the capacitive current ic is in a direction that causes it todecrease the absolute potential of Vdg such that Vdg 0.5 + icR2 . Thus,Vcg Vdg . This voltage imbalance gives rise to transient distortion ofthe output.
 2. The transformer circuit of claim 1 further includingapplied capacitance between said first and third conductors tosubstantially eliminate the resultant capacitive current in said thirdconductor caused by the mutual stray capacitance between said first andthird conductors.
 3. The transformer circuit of claim 1 furthercomprising shielding means surrounding said transformer circuit andelectrically connected to one end of said first conductor and saidreference source, and connector means for permitting electricalconnections through said shielding means to said circuit.
 4. Thetransformer circuit of claim 3 further comprising a fourth conductorsurrounding said third conductor, one end of said fourth conductor beingelectrically connected to said shielding means and means for connectingthe other ends of said first and fourth conductor to said connectormeans.